The principle of the silver complex diffusion transfer process is described in British Pat. No. 614,155 filed Nov. 2, 1939. This process comprises the steps of exposing a photosensitive element containing a silver halide emulsion layer, developing the exposed photosensitive silver halide emulsion layer and forming a soluble silver complex of unexposed silver halide by treating the said photosensitive silver halide emulsion layer with an alkaline processing fluid in the presence of a developing agent and a silver halide complexing agent, transferring said soluble silver complex by diffusion to the silver receptive layer of an image-receiving element in superposed relationship with said silver halide emulsion, forming at said silver receptive layer an image incorporating silver from said silver complex under the action of development nuclei, and separating said image-receiving element from said photosensitive element. Certain compounds are now conventionally used in such non-light-sensitive image-receiving layers; for example 2-phenyl-5-mercapto-oxadiazole and 5-methylbenzotriazole. These compounds are utilized to control the density and tone of the positive image.
Other toners, such as those described in British Patents 950668 and 1158479, can either accelerate the production of a positive image as compared to an image-receiving layer having no toner added thereto, or as compared to an image-receiving layer with known development retarding toner, for example 1-phenyl-2-tetrazoline-5-thione.
All these compounds, although efficacious in their way, have drawbacks and accordingly the need exists to improve the performance of non-light-sensitive image-receiving layers and in particular to improve the speed of development which would allow a faster "strip time" and improve resolution and exposure latitude by decreasing sideways diffusion of complexed silver.
From U.S. Pat. No. 4,500,632 it is known that S-thiuronium alkyl sulfonates stabilize silver images formed in a photographic light-sensitive material against long term deterioration.